fix(google): support Gemma 4 thinking levels

[dawNotPoi]

Summary

• route Gemma 4 through the thinking-level configuration path without enabling Gemini-3-only behavior
• preserve include_thoughts and reject unsupported budget-only configurations with an actionable error
• validate the Gemma 4 minimal/high levels and cover the final generated request configuration

Verification

• uv run pytest -q tests/test_google_thought_signatures.py tests/test_plugin_google_llm.py (76 passed)
• uv run ruff format --check on changed files
• uv run ruff check on changed files
• uv run --group typing mypy --platform linux -p livekit.plugins.google

Fixes #7071

[longcw]

closing in favor of #7095 with some clean up and validation, you're credited there as co-author